.TH std::unordered_map::find 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::unordered_map::find \- std::unordered_map::find

.SH Synopsis
   iterator find( const Key& key );             \fB(1)\fP \fI(since C++11)\fP
   const_iterator find( const Key& key ) const; \fB(2)\fP \fI(since C++11)\fP
   template< class K >                          \fB(3)\fP \fI(since C++20)\fP
   iterator find( const K& x );
   template< class K >                          \fB(4)\fP \fI(since C++20)\fP
   const_iterator find( const K& x ) const;

   1,2) Finds an element with key equivalent to key.
   3,4) Finds an element with key that compares equivalent to the value x. This
   overload participates in overload resolution only if Hash::is_transparent and
   KeyEqual::is_transparent are valid and each denotes a type. This assumes that such
   Hash is callable with both K and Key type, and that the KeyEqual is transparent,
   which, together, allows calling this function without constructing an instance of
   Key.

.SH Parameters

   key - key value of the element to search for
   x   - a value of any type that can be transparently compared with a key

.SH Return value

   An iterator to the requested element. If no such element is found, past-the-end (see
   end()) iterator is returned.

.SH Complexity

   Constant on average, worst case linear in the size of the container.

.SH Notes

           Feature-test macro          Value    Std                Feature
                                                      Heterogeneous comparison lookup
   __cpp_lib_generic_unordered_lookup 201811L (C++20) in unordered associative
                                                      containers; overloads (3,4)

.SH Example


// Run this code

 #include <cstddef>
 #include <functional>
 #include <iostream>
 #include <string>
 #include <string_view>
 #include <unordered_map>

 using namespace std::literals;

 struct string_hash
 {
     using hash_type = std::hash<std::string_view>;
     using is_transparent = void;

     std::size_t operator()(const char* str) const        { return hash_type{}(str); }
     std::size_t operator()(std::string_view str) const   { return hash_type{}(str); }
     std::size_t operator()(std::string const& str) const { return hash_type{}(str); }
 };

 int main()
 {
     // simple comparison demo
     std::unordered_map<int,char> example{{1, 'a'}, {2, 'b'}};

     if (auto search = example.find(2); search != example.end())
         std::cout << "Found " << search->first << ' ' << search->second << '\\n';
     else
         std::cout << "Not found\\n";

     // C++20 demo: Heterogeneous lookup for unordered containers (transparent hashing)
     std::unordered_map<std::string, size_t, string_hash, std::equal_to<>> map{{"one"s, 1}};
     std::cout << std::boolalpha
         << (map.find("one")   != map.end()) << '\\n'
         << (map.find("one"s)  != map.end()) << '\\n'
         << (map.find("one"sv) != map.end()) << '\\n';
 }

.SH Output:

 Found 2 b
 true
 true
 true

.SH See also

   count       returns the number of elements matching specific key
               \fI(public member function)\fP
   equal_range returns range of elements matching a specific key
               \fI(public member function)\fP
